1. Field of the Invention
This invention relates to pressurized water reactor nuclear power plants, and more particularly to systems for cooling and purifying coolant in the spent fuel pit, refueling cavity and refueling water storage tank, and for transferring coolant between these components.
2. Background Information
In pressurized water reactor nuclear power plants, coolant in the form of light water is passed through fuel assemblies in a reactor core housed in a reactor vessel. The coolant is circulated in a primary loop between the reactor core and a steam generator which utilizes the heat removed from the reactor core by the coolant in the primary loop to generate steam in a secondary loop which drives a turbine generator to produce electricity.
Periodically, the reactor core must be refueled. This is accomplished by flooding a refueling cavity over the reactor vessel with coolant from a refueling water storage tank. The fuel assemblies to be replaced are lifted from the reactor core into the refueling cavity and, while remaining immersed in coolant, are conveyed through a transfer tube and a fuel transfer canal to a spent fuel pit outside containment for on-site storage. A spent fuel pit cooling system removes decay heat released by the stored fuel assemblies. The cooling system pumps coolant from the spent fuel pit through heat exchangers which dump the decay heat to a component cooling water system. The current spent fuel pit cooling systems are required by regulation to be safety grade and that includes energization of the cooling system pumps by a safety grade bus. In order to meet these requirements, two completely separate spent fuel pit cooling systems are provided.
During refueling operations, the operator manipulates a refueling machine by visual observation of the fuel assemblies through the coolant. The operators ability to do so is dependent on the clarity of the coolant. Presently, the refueling cavity is flooded with coolant from the refueling water storage tank through the reactor core. Debris and sediment dislodged from the core leads to poor clarity of the coolant in the refueling cavity. Often auxiliary purification equipment in the form of filters, dimineralizers and skimmers must be brought in to improve coolant clarity during refueling.
Clarity can also be a problem in the spent fuel pit. Presently separate purification systems, independent of the spent fuel pit cooling system, are used to improve the clarity of the coolant in the spent fuel pit. Separate skimmers are also used to remove foreign matter from the surface of the spent fuel pit.
All of these separate, and in some cases redundant, cooling and purification systems require a great deal of equipment including six or seven pumps some of which must be safety grade.